1. Field of the Invention
The present invention relates to an aspherical ophthalmic lens, and in particular, to the surface shape of the first surface of such a lens that has a negative refractive power.
2. Description of Related Art
Spherical surfaces are conventionally used on the first refractive surface (the surface on the side of the lens opposite the eye, i.e., the front refractive surface) of ophthalmic lenses that are used to correct refractive errors of the eye. A spherical surface is used because it is easy to manufacture. On the second refractive surface (the surface on the same side as the eye, i.e., the rear refractive surface), toric surfaces, as well as spherical surfaces, are used to correct astigmatism and other refractive errors. Hereinafter, a lens on which a spherical surface is used as the first surface will be referred to as a spherical lens, and a lens on which an aspherical surface is used as the first surface will be referred to as an aspherical lens.
Generally, the refractive power of a lens is expressed in units of diopters (D). The refractive power at the lens surface (the surface refractive power SRP) is defined in terms of the surface curvature .rho. (in units of m.sup.-1), the radius of curvature R (where R=1/.rho.) and the refractive index n of the lens material as EQU SRP(D)=(n-1).times..rho.=(n-1/R).
The refractive power of the first surface of the lens is hereinafter referred to specifically as the base curve. The curvature corresponding to the base curve is hereinafter referred to as the base curve curvature.
The refractive power of the lens is primarily determined by the refractive powers of the first and second refractive surfaces. Therefore, various values of the base curve can be used to obtain a desired lens refractive power, depending upon how the two surface refractive powers are combined. In practice, however, the base curve is limited to a characteristic range for the refractive power of the lens. By using a characteristic base curve, optical performance is ameliorated because the astigmatic aberration effect on the eye that results from viewing objects through sides of the lens that are spaced from the optical axis is reduced.
Generally, the known solution for minimizing the astigmatic aberration of an ophthalmic lens is referred to as Tscherning's ellipse. Tscherning's ellipse provides a hypothetical solution to the problem for a thin lens. In an actual lens, because the design accounts for the actual path of the light rays (i.e., the so-called ray trace) due to the lens center thickness, the actual solution is slightly different from the hypothetical solution. Nevertheless, the hypothetical solution provides an accurate approximation of the actual solution.
According to Tscherning's ellipse, the optimum base curve to minimize astigmatic aberration differs for far-range viewing and close-range viewing. In other words, the optimum base curve differs according to whether the lens is designed for far-range or close-range viewing. When far-range viewing and close-range viewing are equally emphasized (i.e., given the same weight in the calculations), values of the required base curve can be interpolated from the far-range vision base curve values and the close-range vision base curve values.
As a result, three conceivable designs exist, depending upon whether far-range viewing, close-range viewing or both are considered important. For the present invention, a design for far-range viewing and a design for close-range viewing will be described. A design that accounts for far- and close-range viewing equally, however, can be determined by those of ordinary skill in the art as a variant of these two designs.
One disadvantage of a lens having a negative refractive power that is used primarily for myopia correction is that, as the refractive power becomes stronger, the lens edge thickness (the thickness at the border of the lens) increases.
FIG. 5 shows a lens surface shape of a conventional spherical ophthalmic lens that has been designed for far-range viewing (infinitely far). The refractive power of the lens shown in the drawing is -4.0 D, and the lens diameter is 70 mm. This lens is a commonly used plastic lens with a refractive index of 1.50. The base curve is 4.5 D, and the center thickness is 1.0 mm. In the case of this conventional example, the lens edge thickness ed is 6.9 mm, and the total thickness t of the lens from front to rear is 12.6 mm. As a result, when the lens is used as an ophthalmic lens, the edge thickness is thick and undesirably noticeable. In this example, the radius of curvature R1 of the first surface (the surface to the left of the drawing) is 111.111 mm, and the radius of curvature R2 of the second surface (the surface to the right of the drawing) is 58.730 mm. As is known, the lens edge thickness can be reduced by decreasing the base curve.
FIG. 6 shows the lens surface shape of a lens having the same refractive power as the lens of FIG. 5 (-4.0 D), but a base curve of 1.5 D. In this example, the lens edge thickness ed is 6.2 mm, which is 0.7 mm thinner than the lens of FIG. 5. The total thickness t of the lens from front to rear is 8.0 mm, which is 4.6 mm thinner than the lens of FIG. 5. In this example, the radius of curvature R1 of the first surface is 333.333 mm, and the radius of curvature R2 of the second surface is 90.884 mm. As previously stated, however, because the base curve itself is established from the standpoint of conventional optical performance, the low base curve value of 1.5 D in this example results in poor optical performance.
FIGS. 7 and 8 show astigmatism in the field of view when lenses having base curves of 4.5 D and 1.5 D, respectively, are used. The vertical axis shows the angle of the field of vision (units of .degree.), and the horizontal axis shows the astigmatism (units of D, the difference (m-s) between the meridional direction (m) and the sagittal direction (s)), taking the refractive power on the optical axis as the standard.
As shown in FIG. 7, in the lens with a base curve of 4.5 D, the astigmatism is desirably reduced over virtually the entire field of vision. Conversely, as shown in FIG. 8, in the lens with a base curve of 1.5 D, the astigmatism increases significantly toward the periphery of the field of vision. Therefore, FIGS. 7 and 8 show how selecting a base curve affects the final optical performance.
FIG. 9 shows the lens surface shape of a conventional spherical ophthalmic lens that is based on the close-range (30 cm) design. The refractive power of the ophthalmic lens shown is -4.0 D, and the lens diameter is 70 mm. This lens is a commonly used plastic lens with a refractive index of 1.50. The base curve is 3.0 D, and the center thickness is 1.0 mm. In the case of this conventional example, the lens edge thickness ed is 6.5 mm, and the total thickness t of the lens from front to rear is 10.2 mm. As a result, when the lens is used as an ophthalmic lens, the edge thickness is thick and undesirably noticeable. In this example, the radius of curvature R1 of the first surface is 166.667 mm, and the radius of curvature R2 of the second surface is 71.367 mm. As discussed above in connection with the lens of FIG. 5, the base curve can be reduced to decrease the lens edge thickness.
FIG. 10 shows the surface shape of a lens that has the same refractive power as the lens of FIG. 9 (-4.0 D) and a base curve of 0.5 D. In this case, the lens edge thickness ed is 6.0 mm, which is 0.5 mm thinner than the lens of FIG. 9. The total thickness t of the lens from front to rear is 6.7 mm, which is 3.5 mm thinner than the lens of FIG. 9. In this example, the radius of curvature R1 of the first surface is 1000 mm, and the radius of curvature R2 of the second surface is 111.107 mm. Because the base curve is determined based upon conventional optical performance, however, the low base curve value of 0.5 D results in poor optical performance.
FIGS. 11 and 12, which are similar to FIGS. 7 and 8, show astigmatism in the field of vision when lenses of 3.0 D and 0.5 D, respectively, are used. As shown in FIG. 11, in the lens with a base curve of 3.0 D, the astigmatism is desirably reduced over virtually the entire field of vision. Conversely, as shown in FIG. 12, in the lens with a base curve of 0.5 D, the astigmatism increases significantly toward the periphery of the field of vision.
Several methods exist for addressing the undesirable external appearance and poor optical performance in a lens with a negative refractive power used for myopia correction. These methods require using at least one aspherical surface as the first refractive surface or the second refractive surface of the lens. Examples of aspherical ophthalmic lenses having an aspherical first refractive surface are disclosed in Japanese Laid-open Patent Applications Sho 53-94947 and Hei 2-289818 and U.S. Pat. No. 4,279,480.
In the aspherical ophthalmic lens of Japanese Unexamined Patent Application Sho 53-94947, the first refractive surface is divided into a central component (having a diameter of 40 mm) and a peripheral component. The central component acts as one spherical surface, and the peripheral component is structured as a ring with a curvature larger than the curvature of the central spherical surface. In the case of this lens, because the comparatively large central component occupies the center, a significant difference in the curvature at the center cannot be accommodated because the optical performance of the peripheral component will be adversely affected. As a result, the ophthalmic cannot be made as thin as desired. Thus, the ophthalmic lens cannot be made very thin.
The aspherical lens disclosed by Japanese Unexamined Patent Application Hei 2-289818 aims to achieve both sufficient optical performance and a desirable external appearance. However, although the aspherical lens obtains a somewhat suitable result, the optical performance is still not sufficient.
In the aspherical ophthalmic lens disclosed in U.S. Pat. No. 4,279,480, the profile of the first refractive surface is given by a characteristic function. The particular characteristic function disclosed produces a slight concavity near the center of the first refractive surface of the lens. Because unequal reflection occurs at the first refractive surface, the lens appears to undulate. As a result, the external appearance of the lens is undesirable.
Aspherical ophthalmic lenses having an aspherical second refractive surface are disclosed, e.g., in Japanese Laid-Open Patent Applications Sho 53-84741, Sho 53-85742, Sho 58-195826 (corresponding to IT48315/82), and Sho 60-60724. The common disadvantage of these ophthalmic lenses is that because the first refractive surface on a lens with an astigmatism is a convex toric surface or a cylindrical surface, a poor external appearance results when a lens having an aspherical second refractive surface is used. In addition, on ophthalmic lenses that are currently used, the second refractive surface is formed as a concave toric surface, and the lens manufacturing machinery is designed accordingly. Therefore, fabricating lenses with an aspherical second refractive surface would require large-scale changes in lens manufacturing facilities.